The emissivity of electrodes of discharge lamps has a decisive influence on the performance and the geometrical design of such discharge lamps.
The prior art is represented by paste coating with metal powders or mixtures of materials by means of an organic binder and subsequent sintering in or caking onto the electrode body. However, the layer which has been pasted and sintered in is less resistant in mechanical terms, and this can lead to partial crumbling upon contact.
WO 2008/090030 A1 discloses a method for processing an electrode of a discharge lamp. In this case, the electrode is oxidized in the region in which it is pinched in a gas-tight manner in the neck of a discharge space formed from glass. The oxidation is effected chemically in a normal air atmosphere and at ambient air pressure at a temperature of between 700 and 1300 K. The oxide layer is then sublimated in a vacuum environment, the temperature during the sublimation being between 1450 K and 1900 K. This procedure provides the electrode with a surface of fine roughness in said region, as a result of which the adhesion of the surface of this electrode portion to the discharge vessel material is reduced. As a result, the risk of cracking in the sealed region of the discharge vessel is reduced. During the sublimation step, possible contamination is also removed from the surface of the electrode portion with the oxide layer, as a result of which the adhesion is likewise reduced.
U.S. Pat. No. 6,626,725 B1 discloses a discharge lamp in which a rod-shaped electrode consisting of tungsten is introduced in certain regions into a neck of a discharge vessel through a gas-tight pinch seal and extends in certain regions into a discharge space of the discharge vessel. In order to make it possible to prevent cracking of the discharge vessel in the region of the pinch seal during operation of the discharge lamp, the surface of the electrode is processed. To produce an elemental tungsten layer on the surface of the electrode in the length region in which the electrode is arranged in the region of the pinch seal, an oxide layer is firstly produced on the surface. In this respect, a tungsten trioxide layer can be produced, for example. In order to produce the elemental tungsten layer, the oxidized electrode is then heated at about 1200° C. in a hydrogen furnace, in which hydrogen bubbles through water.
EP 1 251 548 A1 teaches a method for improving the thermal radiation properties of electrodes in a high-pressure discharge lamp of the short arc type. For this purpose, grooves are made in the surface of the electrodes. The grooves have a depth which is less than/equal to 12% of the electrode diameter, the ratio between the depth and the spacing of the grooves being greater than/equal to two. A laser apparatus can be used for making the grooves. The grooves can have an angular or curved form, with curved grooves being produced by grinding the surface and then electrolytically polishing it in a 10% strength sodium hydroxide solution. Curved grooves can, however, also be produced by heating to a high temperature in a vacuum, for example by heating the surface at 2000° C. for 120 min.